1. Field of the Invention
This invention relates generally to a power steering system for a vehicle and, more particularly, to a system and method for determining the optimum speed of a magneto-rheological power steering pump used in a power steering assembly for a vehicle for improving the efficiency of the power steering assembly.
2. Description of the Related Art
It is known in the art to provide a power steering system for a motor vehicle to assist a driver in steering the vehicle. Typically, the power steering system employs hydraulics. The hydraulic power steering system typically employs an engine driven hydraulic power steering pump for generating pressurized fluid that is coupled to a hydraulic steering gear of the motor vehicle. Since the power steering pump is driven directly by the engine using a belt, its speed is determined by the engine and it operates continuously as long as the engine is running, resulting in continuous losses due to constant circulation of the hydraulic fluid through the steering gear. Continuous operation of the power steering pump at speeds dictated by the engine speed, even when no steering assist is required, results in increased fuel consumption. In addition, the power steering pump is calibrated to provide the required flow and pressure for the worst case engine speed, which could be near idle, under static steering conditions. Such calibration of the power steering pump results in a much higher pump flow at higher engine speeds further increasing the losses in the hydraulic power steering system, which also results in increased fuel consumption.
More recently, electro-hydraulic power steering systems have been used to decouple the power steering pump from the engine and provide an on-demand hydraulic pressure using an electric motor to drive the hydraulic power steering pump. One known electro-hydraulic power steering system incorporates a hydraulic power steering pump driven by a brushless direct current electric motor controlled by a pulse width modulated inverter. Also, there are electrically driven steering systems that do not use any hydraulic fluids. However, the electro-hydraulic power steering system requires a costly high power electric motor, power electronics for controlling the speed of the electric motor, and a reliable electrical power supply including an engine driven alternator and battery. Further, such systems have high overall losses including losses through the engine alternator, power electronics, electric motor and power steering pump.
Also known in the art is a magneto-rheological hydraulic power steering (MRHPS) system that minimizes power losses in a power steering pump and provides variable flow and pressure of the pump independent of engine speed. The MRHPS system for a vehicle controls the pressure of the steering system using a power steering pump. However, this system does not provide control of steering effort as a function of vehicle speed resulting in a consequent loose feeling of the steering at high speeds and lesser steering assist at lower speed. Further, the system does not provide precision on-center handling feel required at higher pump speed even when the hand-wheel angle rate is very small. Also, the system does not provide uninterrupted operation by managing various possible fault conditions occurring in it.